Developed and application of Planar Optical Circuits (PLC: Planar Lightwave Circuit) have been advanced mainly in quartz-based types, and they have played an important role as a key component underpinning recent optical communication markets in Array Waveguide Gratings (AWG) or splitters among other things. Recently, development of a new functional element has also been advanced, such as a wavelength variable light source having a compound semiconductor amplifier (SOA: Semiconductor Optical Amplifier) hybrid-mounted on a quartz-based PLC. It has been actively explored to mount an active element and a passive element together on a common PLC substrate, to implement a small-sized inexpensive system on one chip.
However, with required functions being increasingly complicated and advanced, the size of elements and driving electrical-power-consumption in PLCs has increased, and limitations have emerged in functions and performances achievable by using quartz-based types. For this reason, research and development of SOI (Silicon on Insulator) waveguides utilizing silicon microfabrication technologies such as silicon thin-wires and Photonic Crystals (PCs) has attracted attention, and their feasibility is examined for key components having small-sized, low electrical-power-consuming, and inexpensive characteristics.
In SOI waveguides, silicon was used as a core material to increase the difference of specific refractive index from that of cladding materials (SiO2 and its dielectrics), to achieve miniaturization. Quartz-based waveguides have a difference of specific refractive index Δ of about 5% and a bending radius of about 500 μm, while Δ of silicon thin-wire optical waveguides is 40% or more and their bending radius can be decreased down to a few μm. For this reason, silicon thin-wires enable significant reduction in the PLC size.
However, when the difference of specific refractive index is increased, the core diameter must be decreased to satisfy a single mode condition for propagating light, and this causes a difference of spot size from that of other waveguide elements such as optical fiber, resulting in increase in loss of optical coupling. In view of this, PTL1 and PTL2, for example, disclose a technology for enlarging the spot size. In PTL2, the disclosed technology enlarges the spot size by forming an output/input region of a silicon thin-wire optical waveguide into a tapered shape such that the width and thickness of the core layer are each decreased in a direction of light propagation.